1. Field of the Invention
The present invention relates to rail clamps and, in particular, to spring actuated, hydraulically released, brakes for cranes and other material handling equipment.
2. Description of the Related Art
Rail clamps are well known. U.S. Pat. No. 3,972,392 issued Aug. 3, 1976 to Norman Allen Johnson, discloses a fail-safe rail clamping apparatus which is adapted to grip the sides of a crane rail. A pair of clamp levers are provided with brake shoes, at first ends thereof, to allow for clamping engagement with opposite sides of the rail. Second ends of the clamp levers are pivotally connected to actuating links to form a toggle mechanism. A compression spring acts on the toggle mechanism to pivot the clamp levers so as to urge the first ends of the clamp levers into engagement with the rail. However, the mechanism ratio of the clamp levers and the toggle mechanism changes faster than the spring force. Accordingly, for narrow rails or for worn brake shoes, the clamping force be very high even for a very small toggle angle.
It is also known to use a wedge to pivot a pair of clamp levers in order to effect braking. U.S. Pat. No. 5,353,895 issued Oct. 11, 1994 to Dermot E. Camack et al., discloses a spring actuated, hydraulically released, emergency braking system for hoist cages. The braking system comprises a pair of brake arms with opposed brake shoes at first ends thereof, and cam followers at second ends thereof. Hydraulic pressure, normally applied to a cylinder, maintains a piston rod in an extended position with the cam followers riding on a cam and the brake shoes clear of the braking surfaces. Upon release of the hydraulic pressure, the piston rod is retracted by a spring force. This causes the cam followers to travel up the cam surfaces and the brake levers to pivot into engagement with the braking surfaces. However, once the brake application commences and the spring force decreases, the cam followers continue to travel along the cam surfaces. Accordingly, the braking force decreases due to decreased spring force resulting from extension of the spring.